Earthquakes are caused by a sudden release of built up energy created from tectonic forces within the Earth. The starting point of an earthquake is known as its epicenter. The strength of the earthquake dissipates as its waveforms radiate away from the epicenter. Most epicenters occur within the Earth's crust, being located approximately 40 km (25 mi) below the Earth's surface.
Detection of earthquakes is accomplished using seismometers, also known as seismic sensors. Seismic sensors detect ground motion by identifying the waveform of the motion and its magnitude. The magnitude measures the energy released at the source of the earthquake and is often expressed using the Richter scale. Richter scale numbers are calculated on prior/historical data, after the earthquake has occurred. However, real-time calculations must still be made based on the measured acceleration of the earthquake waveform at each location. Most destructive earthquakes typically have magnitudes between about 5.5 and 9 on the Richter scale.
The earthquake waveforms are generally of two main types: a Primary Wave (P Wave) and a Secondary Wave (S Wave). P waves are compression waveforms that shake the ground back and forth in the direction that the waveform is moving, where the displacement of the medium is in the same direction as the direction in which the waveform propagates. S waves, also known as shear or transverse waves, create oscillations that occur in a direction that is perpendicular to the direction of waveform motion/energy transfer. P waves travel fastest, are perceived first and arrive before S waves, and typically cause minimal damage. P waves have a velocity that typically ranges from 5 to 8 km/sec (3 to 5 mi/sec). S waves, in contrast, are more destructive than waves, travel at approximately 60% the velocity of the P waves, and can move only through solids.
For the real time earthquake calculations, magnitudes of ground acceleration are also used as means to measure the intensity of an earthquake. The magnitude of acceleration of an earthquake is typically measured in G's or in m/s2. Typical values for the magnitude of acceleration associated with the relative intensity of an earthquake include: 0.001 G (0.01 m/s2), which is associated with an earthquake that people can perceive; 0.02 G (0.2 m/s2), which is associated with an earthquake in which people can lose their balance; and 0.50 G (0.5 m/s2), which is associated with a high intensity earthquake that causes significant damage to most buildings lacking adequate earthquake-resistant features and endangers personnel safety. However, buildings that are either originally designed with such protective features or have the features added to the buildings often incur minimal damage if the earthquake is short in duration.